(1) In an aspect, this invention relates to a receiver and, more particularly, to a receiver applicable to an automobile audio tuner system.
The trend of electronization of automobiles seems to be practically limitless. In recent years, a so-called keyless entry system has become popular to allow the automobile driver to remotely lock and unlock the door of the automobile.
FIG. 1 of the accompanying drawing is a schematic block diagram of the tuner section of a known keyless entry system.
Referring to FIG. 1, tuner 100 comprises an RF (radio frequency) amplifier 102 and a keyless signal generating circuit 103, wherein a signal wave is emitted from the door key as a switch arranged at the door key 104 is depressed. The signal wave is then entered to the RF amplifier 102 by way of an antenna 101 arranged near the transmission system or the fender of the car. The signal wave is then amplified by the RF amplifier 102 for high frequency amplification. The keyless signal generating circuit 103 generates a keyless signal according to the output signal of the RF amplifier 102.
The keyless signal is then entered to microcomputer 105 of the keyless entry system. Upon receiving the keyless signal, the microcomputer 105 controls the door lock for locking and unlocking the door. More specifically, the microcomputer 105 recognizes the status of door lock mechanism 106 of the automobile so that it unlocks the door when it receives a keyless signal while the door is held in a locked state by the door lock, whereas it locks the door when it receives a keyless signal while the door is held in an unlocked state by the door lock.
The tuner of a keyless entry system is either of an FM tuner system that utilizes FM signals or of an AM tuner system that utilizes AM signals.
Referring to FIG. 2 of the accompanying drawing, the keyless signal generating circuit 103 of an FM tuner system typically comprises a converter 1033 for frequency conversion including a mixer 1031 and an oscillator circuit (OSC) 1032, an IF (intermediate frequency) limiter amplifier 1034 and an FM detector circuit 1035.
Referring to FIG. 3 of the accompanying drawings, the keyless signal generating circuit 103 of an AM tuner system, on the other hand, typically comprises a converter 1038 for frequency conversion including a mixer 1036 and an oscillator circuit (OSC) 1037, an IF amplifier provided with an AGC (automatic gain control) function and an AM detector circuit 10310.
Meanwhile, an automobile is provided with an mobile radio receiving set as a standard or optional equipment. Such a mobile radio receiving set comprises an FM (frequency modulation) tuner and an AM (amplitude modulation) tuner.
FIG. 4 of the accompanying drawing is a schematic block diagram of the FM/AM tuner section of a mobile radio receiving set.
Referring to FIG. 4, the tuner 200 comprises an RF antenna for FM broadcast, a regenerative circuit 203 for FM broadcast and a stereophonic demodulation circuit 204.
The FM tuner section of the tuner 200 includes an RF amplifier 202 for FM broadcast, a regenerative circuit 203 for FM broadcast and a stereophonic demodulation circuit 204. An FM wave is fed to the RF amplifier 202 for FM broadcast by way of the antenna 201. Then, the FM wave is amplified for high frequency amplification. The regenerative circuit 203 for FM broadcast generates an FM composite signal on the basis of the output signal of the RF amplifier 202 for FM broadcast. The stereophonic demodulation circuit 204 generates a left/right signal for FM broadcast on the basis of the FM composite signal.
Referring to FIG. 5 of the accompanying drawing, the regenerative circuit 203 for FM broadcast typically comprises a converter 2033 for frequency conversion including a mixer 2031 a VCO 2032 for FM broadcast, an IF limiter amplifier 2034 and an FM detector circuit 2035.
The stereophonic demodulation circuit 204 typically includes a pulse noise reduction circuit and a stereophonic decoder.
The AM tuner section of the tuner 200, on the other hand, includes an RF amplifier 205 for AM broadcast and a regenerative circuit 206 for AM broadcast. An AM wave is fed to the RF amplifier 205 for AM broadcast by way of the antenna 201. Then, the AM wave is amplified by the RF amplifier 205 for AM broadcast. The regenerative circuit 206 for AM broadcast generates an AM radio signal on the basis of the output signal of the RF amplifier 205 for AM broadcast.
As shown in FIG. 6 of the accompanying drawing, the regenerative circuit 106 for AM broadcast typically comprises a converter 2038 for frequency conversion including a mixer 2036 and a VCO 2037 for AM broadcast, an IF amplifier 2039 having an AGC (automatic gain control) function and an AM detector circuit 20310.
As seen from the above description, the known keyless entry system and the automobile audio system have respective histories of technological development. One of the reasons for this may be that the reception frequency (about 300 MHz to 430 MHz) of the tuner of any known keyless entry system is higher than those of the MW band and the VHF band used for the FM/AM tuner of any existing automobile audio system. Therefore, the tuner section of the keyless entry system and the FM/AM tuner section of the automobile audio system have respective configurations that are different from each other in an automobile.
However, referring to FIG. 7 of the accompanying drawings, the signal wave is typically used digitally to express the existence of a signal (xe2x80x9c1xe2x80x9d) and the nonexistence of a signal (xe2x80x9c0xe2x80x9d) in an AM tuner type keyless entry system for both transmission and reception. If such is the case, the tuner section may not need an AGC function because the tuner section can process the received signal wave without distortion.
Therefore, the IF amplifier of an AM tuner type keyless entry system may be replaced by a limiter amplifier that is normally used in an FM tuner type keyless entry system.
Additionally, a keyless entry system comprises and an automobile audio system have components that are common in both of them.
Thus, it is an object of the present invention to provide a tuner section that can be used commonly for a keyless entry system and an automobile audio system in order to reduce the number of components in an automobile equipped with a keyless entry system and hence the manufacturing cost such a keyless entry system.
(2) In another aspect, the present invention relates to a crystal oscillator and, more particularly, it relates to a crystal oscillator to be suitably used for generating a master clock for the tuner and the station selecting PLL (phase locked loop) circuit of a radio or a television receiver.
FIG. 8 of the accompanying drawing is a schematic block diagram of a principal part of a known radio receiver.
The radio receiver comprises an FM tuner, an AM tuner and a PLL circuit.
The FM tuner includes an RF (radio frequency) amplifier 11, a mixer 12 and a band-pass filter 13.
The high frequency signal caught by the antenna 10 of the radio receiver is fed to the RF amplifier 11 and amplified. The output of the RF amplifier 11 is sent to the mixer 12 and the PLL circuit 31 to select a station. The output of the mixer 12 is turned into an IF (intermediate frequency) signal as it passes through the band-pass filter 13.
The AM tuner includes an RF amplifier 21, mixers 22a and 22b, band-pass filters 23a and 23b and a crystal oscillator 24.
The high frequency signal received by the antenna 10 is sent to the RF amplifier 21 and amplified. The output of the RF amplifier 21 is sent to the mixer 22a to select a station. The output of the mixer 22a is sent to the mixer 22b by way of the PLL circuit 23a and subjected to frequency conversion in the mixer 22b. The mixer 22b is fed with the clock generated by the crystal oscillator 24. The output of the mixer 22b is turned into an IF signal as it passes through the band-pass filter 23b. 
The station selecting PLL circuit 31 includes an FM programmable divider 32a, an AM programmable divider 32b, an FM reference divider 33a, an AM reference divider 33b, a phase comparator 34, low-pass filters 35a, 35b, VCOs (voltage controlled oscillators) 36a, 36b and a crystal oscillator 37.
The crystal oscillator 37 generates a clock with a predetermined frequency and the FM reference divider 33a and the AM reference divider 33b generate signals with desired respective frequencies for selecting a station on the basis of the clock with the predetermined frequency.
With a known radio receiver having the above described configuration, the AM tuner needs a crystal oscillator dedicated to the AM tuner. For example, in a double conversion type AM tuner adapted to up-convert the received radio wave to 10.70 or 10.71 MHz and thereafter down-convert it to 450 KHz, it requires a crystal oscillator 24 dedicated to generate a clock (10.25 MHz or 10.26 MHz) to be applied to the mixer 22b. 
Additionally, the station selecting PLL circuit 31 needs a crystal oscillator 37 to generate a clock with a desired frequency for selecting a station. Conventionally, the crystal oscillator of the AM tuner and that of the station selecting PLL circuit may be combined into a single unit to reduce the manufacturing cost thereof. If such is the case, the reference dividers 33a, 33b generate a clock with a desired frequency, using 10.25 MHz or 10.26 MHz.
Conventionally, the FM tuner section 32, the AM tuner section 33 and the station selecting PLL circuit 31 are formed into respective different chips. Meanwhile, in recent years, ICs including those comprising an FM tuner section 32 and an AM tuner section 33 as integral parts thereof, those comprising an FM tuner section 32 and a station selecting PLL circuit 32 as integral parts thereof and those comprising FM/AM tuner sections 32, 33 and a station selecting PLL circuit 31 as integral parts thereof have been developed.
Such one-chip ICs are remarkably advantageous in terms of reducing the manufacturing cost. An IC comprising an AM tuner section 33 and a station selecting PLL circuit 31 in a single chip is particularly advantageous because a single common crystal oscillator can be used for crystal oscillators 24 and 37 to reduce the total manufacturing cost at least by the cost of manufacturing a crystal oscillator.
In such a one-chip IC, however, the higher harmonic of 10.25 MHz or 10.26 MHz generated in the crystal oscillators 24, 37 enters the received FM band to adversely affect the signal receiving operation of the FM tuner.
(1) According to a first aspect of the invention, there is provided a receiver comprising:
a converter for performing frequency conversion on one of an FM signal to be used for a keyless entry system and a high frequency signal of FM broadcast;
an IF limiter amplifier for amplifying an output signal of the converter and subsequently limiting an amplitude of the output signal to a predetermined level;
an FM detector circuit for FM-detecting an output signal of the IF limiter amplifier; and
a microcomputer for controlling the operation of locking and unlocking a door of an automobile on the basis of an FM keyless signal output from the FM detector circuit;
wherein the FM broadcast generated on the basis of a signal output from the FM detector circuit.
According to the invention, there is also provided a receiver comprising:
a first converter for performing frequency conversion on one of an FM signal to be used for a keyless entry system and a high frequency signal of FM broadcast;
an IF limiter amplifier for amplifying an output signal of the first converter and subsequently limiting an amplitude of the output signal to a predetermined level;
a second converter for performing frequency conversion on a high frequency signal of AM broadcast;
an IF amplifier provided with an AGC function for amplifying an output signal of the second converter;
an AM detector circuit for AM-detection;
a selector for applying one of an output signal of the IF limiter amplifier and an output signal the IF amplifier; and
a microcomputer for controlling the operation of locking and unlocking a door of an automobile on the basis of an AM keyless signal output from the AM detector circuit;
wherein the selector selects the output signal of the IF limiter amplifier when the first converter performs frequency conversion on the AM signal and selects the output signal of the IF amplifier when the first converter performs frequency conversion on the high frequency signal for FM broadcast,
and the AM broadcast generated on the basis of an AM radio signal output from the AM detector circuit.
(2) According to a second aspect of the invention, there is provided a receiver comprising:
an FM tuner having an FM mixer for converting the frequency of an FM signal;
a double conversion type AM tuner having a first AM mixer for up-converting the frequency of an AM signal and down-converting the frequency of the AM signal; and
a PLL circuit having a first VCO for generating an output signal to be applied to the FM mixer on the basis of the outcome of comparing the output signal of the crystal oscillator and that of the FM mixer and a second VCO for generating an output signal to be applied to the first AM mixer on the basis of the outcome of comparing the output signal of the crystal oscillator and that of the first AM mixer;
wherein the crystal oscillator includes a first oscillation circuit, a second oscillation circuit having a loop gain higher than the first oscillation circuit and a control circuit for activating at least the second oscillation circuit for a predetermined period of time after the start and inactivating the second oscillation circuit after the predetermined period of time, while keeping the first oscillation circuit activated;
the output terminals of the first and second oscillation circuits being connected commonly and positively fed back.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinbefore.